1. Field of the Invention
The invention relates to a process for preparing butene oligomers from Fischer-Tropsch olefins.
2. Discussion of the Background
Butene oligomers are valuable starting materials for preparing alcohols. Of the oligomeric butenes, the preferred butene oligomers are the isomeric octenes. Isomeric octenes are butene dimers and are therefore also called dibutene.
Dibutene is an isomeric mixture which is formed by the dimerization or codimerization of butenes, i.e., of n-butene and/or isobutene. The dimerization product of n-butene. i.e., 1-butene or 2-butene, is termed di-n-butene. Significant components of di-n-butene are 3-methyl-2-heptene, 3,4-dimethyl-2-hexene and, to a lesser extent, n-octenes. Di-isobutene is the isomeric mixture which is formed by dimerization of isobutene. Di-n-butene, dibutene, and diisobutene are therefore distinguished by their degree of branching. Di-isobutene contains more highly branched molecules than dibutene, and dibutene is more highly branched than di-n-butene.
Dibutene, di-n-butene, and di-isobutene are starting materials for the preparation of isomeric nonanols by hydroformylation to the C.sub.9 aldehyde, followed by hydrogenation to the alcohol. Esters of these nonanols, in particular the phthalic esters, are plasticizers, which are primarily used in poly(vinyl chloride). Nonanols from di-n-butene are more linear (less branched) than nonanols from dibutene, which in turn are more linear than nonanols from di-isobutene. Nonanol esters derived from di-n-butene have, precisely because of their highly linear structure, greater application advantages than the more highly branched esters derived from dibutene- and diisobutene-based nonanols.
As starting materials for the dimerization reactions, butenes can be produced from the C.sub.4 fraction of steam crackers or FC crackers. The C.sub.4 fraction is generally worked up by first removing or separating 1,3-butadiene by selective scrubbing, e.g., using n-methylpyrrolidone. Then, isobutene is typically removed from the C.sub.4 fraction after the 1,3-butadiene has been removed. Isobutene is a desirable and particularly valuable component of the C.sub.4 fraction because it can be reacted with isobutane to give high-octane isooctane or with an alkanol, such as methanol, to give methyl tert-butyl ether (MTBE), which, as an additive to motor gasoline, improves its octane rating. As stated, isobutene is in demand as a cracking product, it is not generally available for an oligomerization process.
After the reaction of the isobutene, n-butene, n-butane, and isobutane remain behind in the C.sub.4 fraction. However, the proportion of n-butene in the cracked products of a steam cracker or an FC cracker is relatively low, on the order of barely 10% by weight, based on the chief target product ethylene. Thus, a steam cracker having the respectable capacity of 600,000 metric t/year of ethylene produces only around 60,000 metric t/year of n-butenes. Although the amount of both n-butene and isobutene could be increased by dehydrogenating around 15,000 metric t/year of n-butane and isobutane that arise in addition to n-butene, it is not feasible, because such dehydrogenation plants require high capital costs and are therefore uneconomic for such a small capacity.
The amount of n-butenes which arise directly in a steam cracker or an FC cracker is therefore insufficient to produce a sufficient quantity of dibutene for a nonanol plant, whose capacity is so high that it could compete economically with the existing large-scale plants for producing important plasticizer alcohols, such as 2-ethyl hexanol. In order to cover the dibutene requirement of a large nonanol plant, n-butene would have to be collected from several steam or FC crackers and jointly oligomerized. Alternatively, an unfractionated C.sub.4 fraction could be collected from several steam or FC crackers and worked up to n-butene on site. However, the transport of liquid gases is expensive and complex safety precautions are required.
On an industrial scale, over 400,000 metric t/year of ethylene and 500,000 metric t/year of propylene are prepared by optimized Fischer-Tropsch processes in the Union of South Africa. A major disadvantage in these processes is the unavoidable production of large amounts of higher hydrocarbons and, in particular, of C.sub.4 hydrocarbons for which, apart from as additive to internal combustion fuels, there is virtually no suitable use. However, even in fuel, the C.sub.4 fraction is somewhat unsuitable because of its high content of olefins, which have a tendency to form gums, and because of its high vapor pressure, which contributes to environmental pollution.
In general, there are no known feasible processes for producing butenes and butane oligomers at a single site in amounts sufficient to supply, e.g., a large nonanol plant.